Thermal-hydraulic Design And Research Of Stirling Heat Exchanger For Molten Salt Space Reactor

Space exploration needs energy support.As one of the space energy sources,space nuclear reactor is free from environmental impact,long-life,safe and reliable,and has become the development trend of space energy.Molten salt reactor,as an important type of the fourth generation reactor,uses molten salt with high boiling point as nuclear fuel.It has many advantages such as high power density,high output temperature,high energy conversion efficiency,simple structure and easy operation,etc.It has great advantages in space energy application and is an ideal energy source for extraterrestrial bases and other deep space exploration missions.In the space reactor system,the function of thermal power conversion system is to convert the fission heat energy generated in the reactor into electric energy,which is an important part of the space reactor energy system.The free piston Stirling engine is an ideal space power generation system with many advantages.In this paper,the application of Stirling and Brayton cycle in molten salt space reactor is analyzed and compared,and the further design and research of Stirling heater is carried out.According to the requirements of molten salt space reactor for dynamic thermal power conversion,the thermodynamic models of Stirling cycle and Brayton cycle are established,analyzed and compared.The conclusion is drawn that the output power of Stirling engine increases first and then decreases with the increase of endothermic temperature,and increases with the increase of exothermic temperature.The cycle efficiency increases with the increase of endothermic temperature and decreases with the increase of exothermic temperature.At the same endothermic and exothermic temperatures,Stirling engines using helium as working medium have higher efficiency than engines using air or hydrogen as working medium.For the Brayton cycle,the output power and efficiency increase first and then decrease with the increase of temperature ratio.Improving the efficiency of the compressor and turbine has a significant effect on the improvement of maximum power and efficiency.By comparing the two cycles,the Stirling cycle is determined as the thermal power conversion method of molten salt space reactor.The heater structure scheme is designed for molten salt space reactor Stirling engine.Different types of fins and molten salt channel structures are designed for the heater from the perspective of enhancing heat transfer capacity,reducing pressure loss and power flattening.The numerical simulation of the heater is carried out using the computational fluid dynamics software Fluent,and the influence of the fin height,pitch,thickness,channel outer diameter,channel type and other parameters on the heater is analyzed.According to the inlet and outlet pressure drop,heat exchange capacity,heat transfer coefficient and heat flux distribution are compared and analyzed for various heater schemes and optimized schemes are obtained.Three schemes are selected for comparative analysis of their applicability at different inlet flow rates.It is concluded that adding fins to molten salt channel can greatly improve the heat transfer capacity of the heater.For radial fins,reducing the fin spacing and thickness can improve the heat transfer capacity,while increasing the fin height has adverse effects on heat transfer.For circumferential fins,increasing fin height and reducing fin spacing can further enhance heat transfer capacity,while reducing fin thickness has adverse effect on heat transfer.Reducing the outer diameter of molten salt channel can also enhance heat transfer to some extent.Compared with finless structure,the maximum increase of heat transfer power and heat transfer coefficient are 76.2% and 68.8% respectively.Compared with the single radial inlet and tangential inlet channel,the double radial inlet and outlet channel structure has relatively uniform heat flux distribution,and the pressure drop is reduced,and the heat transfer capacity is significantly improved.At low flow rates,the double radial inlet and outlet structure with narrow outer diameter and densely packed circumferential fins has better heat transfer capacity.However,in the case of high flow rates,it no longer has performance advantages.The heater structure with radial fins has better heat transfer level and lower pressure drop.The above conclusions can provide some references for the design of molten salt space reactor Stirling heater.